1. Field of the Invention
This invention relates to an optical glass for molding, in particular, an optical glass for a precision molding lens capable of carrying out molding at most 400xc2x0 C.
2. Description of the Prior Art
Of late, aspherical lenses or micro optical lenses used in the optical lens system have often been produced by a molding technique using a high precision metallic mold without polishing. However, the quality of a metallic mold suitable for molding is subject to various limitations from the respect of workability, durability and mass production property. This teaches that the property of a glass to be molded is also limited. The most important property limited is a softening temperature. Molding of a glass having a softening temperature of 600 to 700xc2x0 C. or higher, for example, has a large influence upon the life of a metallic mold and thus results in lowering of the mass production property of lenses. Accordingly, it has been considered difficult from the standpoint of mass productivity to press-mold commercially available optical glasses of all kinds having been marketed and consequently, it becomes a subject of research to develop a glass excellent in press molding property.
IN JP-A-02-124743, for example, there is disclosed a low softening point, medium refractive index and low dispersion optical glass for precision molding lens, having a yielding temperature (At) of at most 500xc2x0 C., refractive index (nd) of 1.53 to 1.62 and Abbe number (xcexd d) of 59.0 to 64.0, and comprising P2O5 and ZnO, as an essential element, and 28 to 49 weight % of ZnO+BaO+SrO+CaO+MgO. This optical glass has such a feature that grinding or polishing after molding is not required because of having a low yielding temperature (At) and excellent stability chemical durability, weather proof property as well as softening property.
JP-A-08-183632 and JP-A-11-139845 have made similar proposals, with which lowering of the softening temperature of the glass is a common subject.
Many of these glasses, however, have softening temperatures of about 400-500xc2x0 C., since if lower than this temperature range, there arises a problem that chemical durability is lowered and no practical glass is obtained. When the composition of such a glass is so selected as to correspond to the optical properties of commercially available optical glasses, a sufficient softening temperature cannot be obtained sometimes. Further, a glass described in JP-A-57-027941 is known as a low softening point glass (having a low glass transition temperature). This fluorophosphate glass is a glass having a glass transition temperature of about 100xc2x0 C., i.e. very low softening point glass, but meets with low productivity because of containing a large amount of fluorides resulting in vaporization of low boiling point fluorides during glass melting. Thus, this glass is not considered suitable for mass production.
Phosphate glasses having relatively a lower softening temperature of oxide glasses have hitherto been proposed as a low softening point glass, for example, as shown in JP-A-60-171244, JP-A-61-036136, JP-A-02-116642, JP-A-02-124743, JP-A-03-040934, JP-A-05-132339, JP-A-08-183632, JP-A-09-278479, JP-A-09-301735, etc.
The inventors have made efforts to develop a glass capable of being subjected to press molding at a low temperature, i.e. at most 400xc2x0 C., in particular, about 380xc2x0 C. in the above described oxide glass compositions of phosphate type. In view of the technical commonsense of the prior art, it is considered to increase the alkali component so as to lower the glass softening temperature, but if the alkali component is merely increased, the stability, chemical durability, weather resistance and softening property of the glass cannot be satisfied.
It is found as a result of our studies that in the above described glass compositions of phosphate type, the above described problem can be solved by incorporating a considerable amount of Al2O3 in an increased amount of Li2O, Na2O and K2O as an essential element and the present invention is based on this finding.
It is an object of the present invention to provide an optical glass for molding, being excellent in workability and capable of carrying out a precision molding at a low temperature, e.g. at most 400xc2x0 C.
It is another object of the present invention to provide, with an excellent producibility, having optical properties, i.e. refractive index (nd) of 1.50 to 1.55 and Abbe number (xcexd d) of 58 to 67.